Simple and inexpensive sensors that can selectively detect carbon monoxide in the presence of other gases, especially other reducing gases, are needed for controlling the exhaust gas emissions of internal combustion engines as well as for other health and safety applications. Metal oxide resistive type sensors and calorimetric-type sensors are presently available for detecting carbon monoxide. However, these sensors are non-selective with respect to other reducing gases. This restriction severely limits the usefulness of these types of sensors for motor vehicle engine applications, because the exhaust gases from such engines contain a wide variety of hydrocarbons and other interfering species along with the carbon monoxide.
It is well-known that the electrical conductivity of many metal oxide sensors may be modified by the adsorption or reaction of gases on their surfaces. This characteristic may be used for the purpose of detecting such gases. Thus, a gas sensor may be prepared, comprising a metal oxide having a surface which can be exposed to an atmosphere containing the particular gas or gases to which the device is sensitive. The sensor typically includes a pair of spaced-apart electrodes in electrical communication with the metal oxide. The amount of electric current which may be passed between the electrodes at a constant applied voltage will be dependent upon the conductivity of the metal oxide. Such devices are typically operable over a particular temperature range and a specific detectable gas concentration range.
U.S. Pat. No. 4,587,104 to Yannopoulos discloses a device for detecting and measuring the amount of carbon monoxide and hydrogen contained in a mixture of reducing gases. The device comprises a bismuth molybdate (Bi.sub.2 O.sub.3.sup.. 3MoO.sub.3) based semiconductor oxide element having two platinum wire electrodes. The electrical conductivity of the device varies in response to the concentration of carbon monoxide and hydrogen in a mixture of reducing gases, but the device is not selective to carbon monoxide alone.
It would be desirable to prepare a sensor which is selective to carbon monoxide. The desired sensor should additionally be useful over a wide range of carbon monoxide concentrations and temperatures.